1. Field of the Invention
The present invention relates to methods of determining time to failure in resins. More particularly, it relates to an accelerated method of determining time to failure of polyethylene resins frequently used in pipe applications.
2. Background of the Art
High resistance to slow crack growth is one of the most important physical properties sought for resins to be used under a variety of conditions, and particularly those that will be used in applications such as pipelines where pressurization can result in dangerous and costly failures. Slow crack growth, which is the slow extension of a crack with time, can significantly reduce the life expectancy of such pipelines, and accurate means of measuring, and therefore predicting, the point of failure are necessary to enable appropriate replacement scheduling.
As used herein, failure refers to brittle failure, which is splitting of a test specimen, or bursting, splitting, or weeping, i.e., seepage of liquid, occurring in an actual pipe or in a pipe specimen, without visible permanent material deformation such as stretching, elongation, or necking down occurring in the area of the break. However, the resin industry has been handicapped in developing and testing new resins for such applications because of the long time periods required to test these materials using many currently-employed methods.
Generally, there are two types of test methods currently being employed. One type involves actually preparing a pipe of the resin and pressurizing it under applicable conditions until failure occurs. These tests are exemplified by methods such as those described by the American Society for Testing Materials (ASTM) in its tests designated as F1474 (also called ISO 13479); ASTM D2837; and ASTM D1598. While these tests are often necessary for actual resin certification, they are expensive and time-consuming, and thus, their use in conjunction with resin development tends to slow down development of new resins. Simply stated, development of new resins is not generally initiated until testing shows that already-developed resins are unsatisfactory, so shortened testing time enables more rapid initiation of development of new resins.
The other type of tests are laboratory scale tests. These include the frequently-used ASTM F1473, also called the Pennsylvania Notched Test, or “PENT,” and the Full-Notched Creep Test, also called “FNCT,” both of which provide faster and more economical ways to assess slow crack growth in resins. However, even these tests present increasingly significant impediments to resin development, as new resins with improved performance exhibit ever-greater resistance to slow crack growth, and therefore extend the time required to accomplish testing.